1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance imaging (tomography) apparatus and to a method, in the form of a pulse sequence, for operating such an apparatus, of the type wherein "in phase" and "opposed phase" gradient echo sequences are used to produce respective images of a subject having different diagnostic content.
2. Description of the Prior Art
As is known, the nuclear magnetic resonance frequency of atomic nuclei is dependent not only on the existing magnetic field but also on the chemical bond of the respective atomic nuclei. This effect is referred to as chemical shift. For example, protons bonded to lipids have a resonant frequency that differs from the resonant frequency of water protons by the chemical shift .delta. of approximately 3.4 ppm. This slight frequency shift leads causes the magnetization vectors of fat and water protons to diverge after an excitation. By contrast to spin echo sequences, dephasing effects that are based on such internal interactions cannot be rephased with gradient echo sequences. For example, a pulse sequence known as the FLASH sequence, as disclosed in U.S. Pat. No. 4,707,658, is a typical representative of gradient echo sequences.
Dependent on the time at which the nuclear magnetic resonance signals are acquired, the signals of water protons and of lipid-bonded protons (referred to in brief below respectively as water signals and fat signals) can constructively or destructively superimpose within a considered voxel. The image appearance is thus critically dependent on the selected echo time of the pulse sequence. The condition wherein fat and water signals are in phase at the echo time, i.e. at the time of the signal acquisition, is referred to as "in phase" imaging; the condition wherein fat and water signals exhibit precisely opposite phases at the echo time is referred to as "opposed phase" imaging.
As explained in the article by N. Rofsky, "Comparison Between In-Phase and Opposed-Phase T1-Weighted Breath-Hold FLASH-sequences for Hepatic Imaging," Journal of Computer Assisted Tomography 20 (2), pages 230 through 235, fat infiltrations in the liver are recognized especially well in "opposed phase" imaging due to the signal loss in voxels that contain water as well as fat protons. Further, a black edge that is likewise to be attributed to the signal loss and can supply valuable diagnostic indications occurs in "opposed phase" imaging at boundary layers between water and fat. It was therefore pointed out in the above reference that "in phase" and "opposed phase" gradient echo sequences supply diagnostic information that supplement one another and that an image according to the "in phase" method and a separate image according to the "opposed phase" method should thereby be considered, for example in imaging the liver.
In order to make these images comparable, of course, no movement on the part of the examination subject should occur between the acquisition of the two images. Both images should therefore be acquired during, for example, a single breath-holding phase.
It is obvious that the probability that motion will occur between the two images, and thus the likelihood that the two images will no longer be spatially congruent, becomes greater the longer the overall data acquisition phase for the two images lasts.
U.S. Pat. No. 4,818,941 discloses a pulse sequence for generating fat and water images that is based on a gradient echo sequence with a readout gradient of alternating polarity. Raw data signals are acquired at two points in time after every excitation. These points in time are selected such that fat and water signals add at one acquisition point and subtract at the other. Pure water or fat images are respectively obtained by addition or subtraction of the data acquired in this way. The data are acquired under pulses of the readout gradient having the same polarity. At least in high-field systems with a minimum basic field strength of 1 Tesla, the pulse of opposite polarity lying between these two data acquisition pulses must exhibit extremely steep edges and a high gradient intensity so that the rephasing condition is satisfied at the predetermined readout times. These demands, however, can be realized if at all only with specially designed gradient amplifiers.